Transcript Darwin`s Theory of Evolution and Natural Selectionx

Food for thought…
 Humans share the Earth with millions of other kinds
of organisms of every imaginable shape, size, and
habitat.
 This variety of living things is called biological
diversity.
 How did all of these different organisms arise?
 How are they related?
What do you think the theory of
evolution means?
 Evolution: (change over time) the process by which
modern organisms have descended from ancient
organisms.
 Scientific theory: a well-supported testable
explanation of phenomena that have occurred in the
natural world.
Charles Darwin
& the Theory of Evolution
 Born in England on February 12, 1809
 The individual who contributed more to our
understanding of evolution than anyone.
H.M.S. Beagle
 In 1831, shortly after college, he joined the crew of the
H.M.S. Beagle and set sail from England for a voyage
around the world.
 This was to be one of the most important voyages in
the history of science.
 During his travels, Darwin made numerous
observations and collected evidence that led him to
propose a revolutionary hypothesis about the way life
changes over time.
Patterns of Diversity
 Darwin visited Argentina and Australia which had
similar grassland ecosystems.
 those grasslands were inhabited by very different
animals.
 neither Argentina nor Australia was home to the
sorts of animals that lived in European grasslands.
Patterns of Diversity
 Darwin posed challenging questions:
 Why were there no rabbits in Australia, despite the
presence of habitats that seemed perfect for them?
 Why were there no kangaroos in England?
Living Organisms and Fossils
 Darwin collected the preserved remains of ancient
organisms, called fossils.
 Some of those fossils resembled organisms that were
still alive today.
Living Organisms and Fossils
 Others looked completely unlike any creature he had
ever seen.
 As Darwin studied fossils, new questions arose:
 Why had so many of these species disappeared?
 How were they related to living species?
The Galapagos Islands
 The smallest, lowest islands were hot, dry, and
nearly barren.
 Hood Island had sparse vegetation
 The higher islands had greater rainfall and a
different assortment of plants and animals.
 Isabela Island had rich vegetation.
The Galapagos Islands
 Darwin was fascinated in particular by the land
tortoises and marine iguanas in the Galápagos.
 Giant tortoises varied in predictable ways from one
island to another.
 The shape of a tortoise's shell could be used to identify
which island a particular tortoise inhabited.
Other animals found in the
Galapagos
 Land Tortoises
 Darwin Finches
 Blue-Footed Booby
 Marine Iguanas
The Journey Home
 While heading home, Darwin spent a great deal of
time thinking about his findings.
 Darwin observed that characteristics of many
plants and animals vary greatly among the islands.
 Hypothesis: Separate species may have arose
from an original ancestor.
Ideas that shaped Darwin’s
thinking
 James Hutton:
 1795- Proposed Theory of
Geological change
 Forces change earth’s
surface shape
 Changes are slow
 Estimates that Earth is
millions-not thousandsof years old
Ideas that shaped Darwin’s
thinking
 Charles Lyell:
 1833-Published book:
Principles of Geography
 Geographical features can
be built up or torn down
over long periods of time.
 Darwin thought if earth
changed over time, what
about life?
Lamarck’s Theory of Evolution
 Lamarck proposed that by selective use or disuse of
organs, organisms acquired or lost certain traits during
their lifetime. These traits could then be passed on to
their offspring. Over time, this led to change in a
species.
 Tendency toward Perfection (Giraffe necks)
 Use and Disuse (fiddler crabs using front claws )
 Inheritance of Acquired Traits
From
repeated use,
claw grows
larger.
Male crab uses front
claw to attract mate and
ward off predators.
The acquired
characteristic,
a larger claw, is
passed on to
offspring.
Population growth
 Thomas Malthus: 19th
century English economist
 If population grew (more
babies born than die):
 Insufficient living space
 Food runs out
 Darwin applied this theory
to animals
Publication of On the Origin of
Species
 Alfred Russel Wallace wrote an essay summarizing
evolutionary change from his field work in
Malaysia.
 This gave Darwin the drive to publish his findings.
Variation and Artificial Selection
 Natural variation: Differences among individuals
of a species
 Artificial selection: Nature provides the variation
among different organisms, and humans select
those variations they find useful.
 Domestic animals and crop plants
Evolution by Natural Selection
 The Struggle for Existence: Members of each species
have to compete for food, shelter, and other life
necessities.
 Faster predators will catch more prey, more camouflaged prey
will avoid being caught
 Survival of the Fittest: Some individuals are better
suited for the environment and therefore better able to
survive and reproduce.
 Adaptations include anatomical or structural characteristics
or behavior
Natural Selection
 Over time, natural selection results in changes in
inherited characteristics of a population. These
changes increase a species fitness in its
environment.
Descent
 Descent with Modification-Each living organism
has descended, with changes from other species
over time
 Common Descent- were derived from common
ancestors
Evidence of Evolution
 The Fossil Record
 Geographic Distribution of Living Things
 Homologous Body Structures
 Similarities in Early Development
Summary of Darwin’s Theory
 Individuals in nature differ, and some of this
variation is heritable.
 Organisms in nature produce more offspring than
can survive, and many of those who do not survive
do not reproduce.
 Because more organisms are produced than can
survive, each species must struggle for resources.
 Each organism is unique, each has advantages and
disadvantages in the struggle for existence.
Summary of Darwin’s Theory
 Individuals best suited for the environment survive
and reproduce most successful
 Species change over time
 Species alive today descended with modification
from species that lived in the past
 All organisms on earth are united into a single
family tree of life by common descent
 Galapagos Islands Activity
Evolution of populations
 In what ways are you like your parents?
 Which traits do you think you inherited?
 So, how do inheritable traits evolve in populations?
Genetic Variation
 Gene pool: consists of all genes, including all different
alleles, that are present in a population.
 Relative frequency: (of an allele) is the number of
times that allele occurs in a gene pool, compared with
the number of times other alleles for the same gene
occur. (percentage)
 In genetic terms: Evolution is any change in the
relative frequency of alleles in a population.
Gene Pool of Fur Color in Mice
 In a total of 50 alleles:
• 20 alleles are B (black): relative frequency = 40%
• 30 alleles are b (brown): relative frequency = 60%
Sources of Genetic Variation
 Mutations
 Genetic shuffling that results from sexual
reproduction
Sickle Cell Anemia and Malaria
 Both sickle cell anemia and malaria are especially
prevalent in areas of Africa.
 Sickle cell anemia is caused by two HbS genes.
 In these areas, carriers of the HbS gene have been
naturally selected because the trait presents
resistance to malaria.
 This immunity is a selective advantage, but also a
disadvantage because the chances of being born with
sickle cell anemia are relatively high.
Single-gene and Polygenic Traits
 The number of phenotypes produced for a given trait
depends on how many genes control the trait.
 Single-gene trait: Controlled by a single gene that has
two alleles
 Polygenic traits: Each gene has two or more alleles
Evolution as Genetic Change
 Natural selection never acts directly on genes.
Why?
 It is an entire organism-not a single gene-that either
survives and reproduces (alleles stay in gene pool) or
dies without reproducing (alleles do not contribute to
gene pool).
Evolution as Genetic Change
 Recall, evolution is any change over time in the
relative frequencies of alleles in a population.
 It is the population, not individual organisms,
that can evolve over time.
Natural Selection on Single-Gene
Traits
 Can lead to changes in allele frequencies and to
evolution.
Natural Selection on Polygenic
Traits
 When traits are controlled by more than one gene,
the effects of natural selection are more complex.
 Natural selection can affect the distributions of
phenotypes in any of three ways:
 Directional selection
 Stabilizing selection
 Disruptive selection
Directional Selection
 When individuals at one end of the curve have higher
fitness than individuals in the middle or at the other
end.
 The range of phenotypes shifts as some individuals fail
to survive and reproduce while others succeed.
Stabilizing Selection
 When individuals near the center of the curve have
higher fitness than individuals at either end of the
curve.
 This keeps the center of
the curve at its current
position but narrows
the overall graph.
 If medical advances could
prevent problems for high
birth weight but not low
birth weight, how might
the curve change?
Disruptive Selection
 When individuals at the upper and lower ends of the
curve have higher fitness than individuals near the
middle.
 Selection acts most strongly against individuals of the
intermediate type.
Population Genetics
 There is MATH to support this!!
 Sex and The Single Guppy online activity